I read that this is weak point, so I was always v. careful with mine. I always pressed the little door home whilst sliding in the locking do-dad.
In spite of this, it started to show stess cracks around the edge of the female bit. I did a fix using polysterene cement (aka airfix glue) and some of that transparent plastic used in blister packs. You'll obviously have to remove some paint before attempting a repair. Not super pretty, but it's a fix non the less.